The Collaborative International Dictionary of English v.0.48:

Revelation \Rev`e*la"tion\, n. [F. r['e]v['e]lation, L.
   revelatio. See Reveal.]
   1. The act of revealing, disclosing, or discovering to others
      what was before unknown to them.
      [1913 Webster]

   2. That which is revealed.
      [1913 Webster]

   3. (Theol.)
      (a) The act of revealing divine truth.
      (b) That which is revealed by God to man; esp., the Bible.
          [1913 Webster]

                By revelation he made known unto me the mystery,
                as I wrote afore in few words.    --Eph. iii. 3.
          [1913 Webster]

   4. Specifically, the last book of the sacred canon,
      containing the prophecies of St. John; the Apocalypse or
      Book of Revelation or The Revelation of Saint John.
      [1913 Webster]

WordNet (r) 3.0 (2006):

revelation
    n 1: the speech act of making something evident [syn:
         disclosure, revelation, revealing]
    2: an enlightening or astonishing disclosure
    3: communication of knowledge to man by a divine or supernatural
       agency [syn: revelation, divine revelation]
    4: the last book of the New Testament; contains visionary
       descriptions of heaven and of conflicts between good and evil
       and of the end of the world; attributed to Saint John the
       Apostle [syn: Revelation, Revelation of Saint John the
       Divine, Apocalypse, Book of Revelation]

The Devil's Dictionary (1881-1906):

REVELATION, n.  A famous book in which St. John the Divine concealed
all that he knew.  The revealing is done by the commentators, who know
nothing.